The present invention relates to a method of feeding, with impure nitrogen, the combustion chamber of a gas turbine combined with an air distillation unit, the air distillation unit being equipped with an apparatus for purifying air of water and of CO2 by adsorption, this apparatus comprising two adsorbers, each one of which follows an operating cycle which comprises, alternately and cyclically, a co-current adsorption phase at a high pressure of the cycle, and a regeneration phase, the regeneration phase comprising a decompression step (a), a step (c) of counter-current purging by waste gas from the air distillation unit coming from a low-pressure column of this unit, and a step (d) of recompressing the adsorber up to the high pressure of the cycle.
The pressures involved here are absolute pressures.
The gas turbines used to drive an alternator for generating electricity are fed by a gas coming from a combustion chamber. This chamber is mainly fed by air compressed by an air compressor, which is, for example, coupled to the turbine, and by a fuel gas. The temperature of the flame, and therefore the generation of NOz, is adjusted by additional injection of nitrogen into the combustion chamber.
The fuel gas is often produced by a gasifier fed, on the one hand, by oxygen, and on the other hand, by carbon products, such as coal. An air distillation unit then produces the oxygen consumed by the gasifier and the nitrogen sent to the combustion chamber.
The waste gas from the low-pressure column of an air distillation unit, for example, a double air distillation column, is impure nitrogen typically containing 0.5 to 1% oxygen, and may therefore provide nitrogen for the combustion chamber, the maximum permissible oxygen content for this gas typically being 2%. In some plants, where all the products from the air distillation unit are utilized under pressure and where the nitrogen flow rate demanded by the gas turbine is high, it is advantageous to choose a low pressure clearly greater than atmospheric pressure (typically 3 to 4 bar, which corresponds to a medium distillation pressure greater than 1C bar), and to purge the purification adsorbers under this low pressure. This is because this makes it possible to recover the gas having carried out the purging and to mix it with impure nitrogen sent directly from the air distillation unit to the gas turbine, without carrying out expansion or recompression which are costly in terms of power.
As is well known, purging the adsorber has the effect of desorbing the contaminants (mainly water and CO2) which have been removed from the air during the previous adsorption phase.
However, this option comes up against a problem at the end of the decompression step, the adsorber still contains much oxygen, such that the gas coming from the adsorber at the start of the purging phase has an oxygen content greater than the maximum permissible content for the feeding of the combustion chamber.
The aim of the invention is to solve this problem, that is to say to make it possible, simply and economically, to recompress the purging gas for feeding the combustion chamber of the gas turbine.
To this end, the subject of the invention is a method of the aforementioned type, characterized in that:
the low-pressure column operates at a low pressure which is clearly greater than atmospheric pressure;
the waste gas is used almost at the low pressure for the purging step (c);
the decompression step (a) is continued down to a minimum pressure of the cycle which is clearly less than the low pressure;
between the decompression step (a) and the purging step (c) a partial repressurization step (b) is inserted, in which the adsorber is repressurized by means of waste gas up to almost the purging pressure; and
the waste gas coming from the adsorber during the purging step (c) is mixed with the waste gas produced by the air distillation unit, and the mixture is compressed by a nitrogen compressor in order to feed the combustion chamber of the gas turbine.
The method according to the invention may comprise one or more of the following characteristics:
the minimum pressure of the cycle is almost atmospheric pressure;
after the partial repressurization step (b), an additional step (bxe2x80x2) of depleting the adsorber of oxygen is carried out at least once, which step comprises the following substeps:
(bxe2x80x21) additional decompression of the adsorber down to the minimum pressure, and
(bxe2x80x22) additional partial repressurization up to the purging pressure,
then the purging step (c) is carried out;
the purging step (c) comprises a substep (c2) during which the waste gas is heated followed by a substep (c3) during which the waste gas is not heated;
a nitrogen-rich gas, having at least the nitrogen content of the impure nitrogen, not coming directly from the low-pressure column, is used in order to carry out the or each partial repressurization step;
the nitrogen-rich gas comes from a nitrogen distribution network; and
the nitrogen-rich gas comes from an auxiliary tank fed by part of the gas delivered by the nitrogen compressor.
The subject of the invention is also an electricity generation plant adapted to implement such a method.
This plant, of the type comprising a gas turbine, an air distillation unit producing impure nitrogen, the air distillation unit being equipped with an apparatus for purifying air of water and of CO2 by adsorption, this apparatus comprising at least two, preferably only two, adsorbers, each one of which follows an operating cycle which comprises, alternatively and cyclically, a co-current adsorption phase at a high pressure of the cycle, and a regeneration phase, the regeneration phase comprising a decompression step (a), a step (c)of counter-current purging by waste gas from the air distillation unit coming from a low-pressure column of this unit, and a step (d) of recompressing the adsorber up to the high pressure of the cycle, and a nitrogen compressor, the intake of which is connected to the air distillation unit by means of a feed pipe and the delivery side of which is connected to the combustion chamber of the gas turbine, in which:
the air distillation unit produces impure nitrogen under a low pressure which is clearly greater than atmospheric pressure;
a first impure nitrogen pipe fitted with a valve connects the air distillation unit to the purified air outlet of each adsorber; and
a second impure nitrogen pipe fitted with a valve connects the air inlet of each adsorber to the feed pipe.